1. Field of the Invention
The present invention relates to a receive path amplifier, and in particular to optimizing gain changes of this amplifier by identifying the modulation type and rate of a received packet.
2. Discussion of the Related Art
In a communication system, receivers are electronic devices that receive incoming signals and are very well known. Certain types of digital receivers have the ability to receive incoming signals that are transmitted with different modulation types. A modulation type generically refers to the type of information that can be added to a signal and to the signal's format. Modulation types can include, for example, Orthogonal Frequency Division Multiplexing (OFDM), Complementary Code Keying (CCK), Discrete Multi-Tone (DMT), and Extended Range (XR). Each modulation type can further include a modulation mapping, which generically indicates how that information is added to the signal. Modulation mappings can include, for example, binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), or quadrature amplitude modulation (64QAM or 16 QAM). Each of these mappings can have an associated modulation rate. Thus, a single receiver can, at different times, receive signals having different modulation types, mappings, and rates.
Generally, it is advantageous to use a more complex modulation mapping if possible, since more information can be communicated for a given bandwidth using a modulation mapping that is more complex. But which modulation mapping to use at different times can depend on various criteria. For instance, when a communication channel is good, the transmitter may try to transmit at a high data rate with a complex modulation mapping, e.g. 64 QAM. However, when the channel is poor, a lower data rate with a less complex modulation mapping, e.g. BPSK, may be used. When switching between modulation mappings, the transmitter will commonly include a header that is modulated using the lower data rate. This header will typically also include an indication of the data rate used for the rest of the packet. A receiver can typically detect and demodulate this lowest common denominator signal at the lower data rate.
Most receivers can provide variable gain, i.e. amplification to an incoming signal, because of possible changes to channel conditions, circuit characteristics, etc. that could necessitate having different gains at different times. Accordingly, variable gain is used to optimize the amplification of the received signals within the dynamic range of the receiver. For example, if the gain is set too low, noise from various amplifiers and mixers in the receiver could be significant compared to the size of the desired signal, thereby degrading performance. A signal to noise ratio (SNR) refers to the ratio of the amplitude of the incoming signal to the amplitude of noise signals at a given point in time. On the other hand, if the gain is set too high, signals in nearby channels could cause the amplifiers and mixers in the receiver to perform non-linearly, e.g. clip or saturate, thereby degrading system performance. Blocker tolerance refers to the ability of a receiver to successfully filter out such nearby signals and still receive the incoming signals. For example, a receiver should still receive incoming signals even if signals 20 MHz away and 16 dB larger than the incoming signals are present. Similarly, a receiver should still receive incoming signals even if signals 40 MHz away and 32 dB larger than the incoming signals are present. Filtering is typically provided by one or more low pass filters in the receiving path to assist in lowering the size of the blocking signals, but this filtering only helps any blocks beyond the filters in the receive chain.
FIG. 1A illustrates the desired SNR as well as blocker tolerance for high bit rate signals and low bit rate signals. A high bit rate is typical of highly modulated signals (e.g. 64QAM), which are generally difficult to receive. Therefore, a high bit rate preferably has an associated high SNR (line 103) that should not exceed a linearity limit (line 101) for the receiver components. In contrast, a low bit rate is typical of less modulated signals and therefore can be easily received but can be more easily influenced by signals in nearby channels. Therefore, a low bit rate preferably has an associated high blocker tolerance (line 104) that should be greater than the ambient noise level (line 102).
Of importance, amplification of the incoming signals is performed before low pass filtering. Therefore, the gain (and thus the signal scaling) should be carefully adjusted in the receive chain. The optimum signal sizing is just large enough to insure that any circuit noise remains small enough relative to the signal size that successful communication can be maintained. By keeping the gain at this lowest allowable level, potentially interfering signals are allowed to be as large as possible without causing the active circuits to saturate.
To optimize the gain, and thus the signal scaling that is appropriate for a given packet, the minimum acceptable SNR can be calculated, simulated, or found from experimental measurements, and then used to determine the gain. As noted above, the required SNR depends on the complexity of the signal being transmitted.
When transmitting signals when using a multi-carrier modulation type, such as OFDM or DMT, there are included intervals when no information is being transmitted, which intervals are expressed as guard times or guard intervals. A guard time exists between each of the transmitted symbols, and is long enough to span the time of the multi-path echoes that will occur in the channel. In the receiver, these guard times are intentionally ignored, so that the multi-path echoes do not corrupt the decoding of the data. These guard periods can be used to adjust the gain in the receiver without causing data loss due to the temporary disruptions that occur when the gain is changed. There are also other times when it may be appropriate to adjust the gain in the receiver, such as when padding bits are being received.
Because the modulation mapping may change in the middle of a packet (at least after the header), scaling the signal to its optimum level is challenging. Because a conventional receiver does not know the modulation format, and thus the data rate of the body of the packet immediately, it cannot know the optimum scaling to use at the beginning of the packet. Therefore, the receiver must be conservative and size the signal large enough so that even if the most complex modulation mapping is used later in the packet, sufficient SNR will exist so that it is received correctly. While this signal sizing will prove correct if the packet really does contain data modulated in the most complex way, if the packet contains data that is modulated in a less complex way, that signal sizing will have been larger than necessary, and sacrifice potential ability to withstand interference from signals in nearby channels. In setting the signal size, consideration must be given to the worst-case power back-off due to the blocker power.
Example of a multi-modulation format signals are the signals associated with the IEEE 802.11a standard or Hiperlan II standard, which each allow for high-speed local area network communications in the 5 GHz communications band. The signal in the 802.11A standard is allocated into one of twelve different 20 MHz channels. Each of the eight channels is divided into 52 different sub-channels or carriers, of which 48 carriers are able to transmit the signal and 4 of the carriers are used to transmit pilot tones. During transmission, the signal is spread onto each of the 48 carriers associated with the channel according to the modulation technique used, and, upon receipt, is de-spread and demodulated to reconstruct the transmitted signal.
FIG. 1B illustrates the beginning portion of a packet for such an OFDM signal 110, which includes ten short training symbols t1-t10, which are identical to each other and used for signal detection, an initial automatic gain control adjustment, diversity selection, coarse frequency offset estimation and timing synchronization. Two long training symbols T1 and T2 that are also identical to each other are typically used for channel and fine frequency offset estimation. Thereafter exists the SIGNAL symbol, which corresponds to the header referred to above, that contains information indicating the data rate at which the following data, illustrated as Data 1, Data 2, . . . , for the remainder of the packet, will be transmitted. In the 802.11a standard, for each different data rate there is a different modulation mapping, which results in a one-to-one correspondence between the data rate and modulation scheme.
FIG. 2 illustrates a functional block diagram of a conventional receiver 200 that can be used to receive signals. Receiver 200 includes a bandpass filter 202 that receives system input signals from an antenna 201 and outputs a predetermined band of frequencies (while excluding those frequencies higher and lower than the predetermined band). A variable RF amplifier 204 provides an initial amplification to that predetermined band of frequencies. A mixer 206 converts those amplified signals into intermediate frequency (IF) signals, which are then amplified by an IF amplifier 208. At this point, mixers 209 and low pass filters 210 (including both I and Q branches) can generate signals in the desired channel (called the baseband signals). Amplifiers 212 then amplify these baseband signals. At this point, analog to digital converters 214 (provided for both the I and Q branches of low pass filters 210) transform the amplified baseband signals into digital signals that can be analyzed by the rest of receiver 200. Of importance, the gains of RF amplifier 202 and IF amplifier 208 are automatically controlled through digital signals. These gains can be computed using various known power estimation algorithms.
A detector 216 and an auto frequency control (AFC) clock recovery circuit 218 ensure an accurate alignment of the baseband signal. Gain control circuit 220 detects the magnitude of the detected baseband signal from detector 216 during the short training symbol sequence thereof and uses the detected magnitude to adjust the gain of RF amplifier 204, IF amplifier 208, and BB amplifiers 212. A signal timing circuit 222, which also receives the output of detector 216, determines those intervals during which an actual symbol exists, rather than a guard interval, and provides a timing output to an FFT 224 (which also receives the output of detector 216). In this manner, FFT 224 can be gated in time to receive the signal data, rather than noise caused by interference that will exist during a guard interval. FFT 224 provides its output to a channel estimation/pilot phase tracking circuit 226 as well as a channel correction circuit 228.
Channel estimation/pilot phase tracking circuit 226 can obtain a channel estimate during the long training symbol sequence, and provide that channel estimate to channel correction circuit 228. Channel correction circuit 228 can then use the channel estimate to compensate for the determined channel characteristics for the rest of the packet. And, if included, a pilot phase tracker will adjust the channel estimate based upon channel information obtained by tracking pilot tones during the transmission of the rest of the packet. The channel corrected signal is then provided to the demapping/interleaving circuit 230 as well as an FEC decoder 232 (typically a Viterbi decoder) for decoding in a conventional manner.
Of importance, in receiver 200, the information in the packet, including the information contained in the SIGNAL symbol, is not available until FEC decoder 232 has completed its operation, which will not occur until after quite a bit of data, as represented by Data 1, Data 2 . . . , has already been received and amplified by RF amplifier 202 and IF amplifier 208.
In operation, the gain used by gain control circuit 220 is initially determined during the initial short symbol training sequence, and then kept constant for the remainder of the packet. Because the gain is held constant, it must be set at a level that allows the sizing of the received symbols to be large enough such that even if the most complex modulation is used in later symbols in the packet, sufficient SNR will exist, thereby ensuring that such symbols can be received correctly.
Other types of receivers operate by continuously varying the gain to insure that whatever signals are received do not overload the receiver. This variance has at least two disadvantages in a packet-based communication system. First, certain undesired signals may come and go abruptly in a packet-based system. Thus, by the time such undesired signals are detected, the desired packet may already be ruined. Second, because it is necessary to change the gain almost immediately to prevent such overload, the gain might need to be changed in the middle of a symbol. However, changing the gain in the middle of a symbol can cause data to be ruined due to the change in signal magnitude and/or phase.
Accordingly, a need arises for a quick and efficient method to control the timing of gain changes, particularly when the modulation type or the modulation rate change.